Regulation of the actin-activated ATPase activity of Acanthamoeba myosin II by copolymerization with phosphorylated and dephosphorylated peptides derived from the carboxyl-terminal end of the heavy chain.

Myosin II from Acanthamoeba castellanii is a conventional myosin composed of two heavy chains and two pairs of light chains. The amino-terminal approximately 90 kDa of each heavy chain form a globular head that contains the ATPase site and an ATP-sensitive actin-binding site. The carboxyl-terminal approximately 80 kDa of both heavy chains interact to form a coiled coil, helical rod (through which the molecules self-associate into bipolar filaments) ending in a short nonhelical tailpiece. Phosphorylation of 3 serine residues at the tip of the tail (at positions 11, 16, and 21 from the carboxyl terminus) inactivates the actin-activated Mg2(+)-ATPase activity of myosin II filaments. Previous work had indicated that the activity of each myosin II molecule in a filament reflects the global state of phosphorylation of the filament rather than the phosphorylation state of the molecule itself. We have now purified the approximately 28-kDa carboxyl-terminal region of the heavy chain lacking the last two phosphorylation sites, and we have shown that this peptide copolymerizes with and regulates the actin-activated Mg2(+)-ATPase activities of native dephosphorylated and phosphorylated myosin II. It can be concluded from these studies that the biologically relevant enzymatic activity of myosin II is regulated by a phosphorylation-dependent conformational change in the myosin filaments.